In the discipline of Ophthalmology one of the diagnostic tests used to test central and peripheral vision is commonly referred to as the "Visual Field Test." This test is performed utilizing a device popularly called a "perimeter." One of the perimeters in wide use and considered by many to be the clinical standard is the Goldmann-type dome perimeter.
Essentially the device consists of a hemisphere shaped dome into which the patient looks. A fixation light is provided to direct the patient's gaze toward the center of the dome while means are provided to locate the eye being tested near the center of curvature of the dome. In the test, stationary (static) or moving (kinetic) light stimuli are presented at various locations on the dome by means of an optical projection system. While gazing at the fixation light, the patient attempts to detect the presence of these stimuli, indicating that he has detected each stimulus by pressing a hand-held button.
In a typical examining procedure the examiner inserts a chart into a frame on the opposite side of the perimeter from the patient. The chart is printed with a pattern which represents the retinal areas of the patient's eye which may be stimulated by projecting the stimulus onto a corresponding location on the dome. The examiner directs the stimulus by means of the projection optics to the desired location on the dome by moving a pantograph arm attached to a pointer over the surface of the printed chart until the point is aligned with the retinal location selected for testing.
Prior to each stimulus or series of stimulus presentations, the operator looks into a telescope provided on the perimeter to make certain that the patient's eye is fixed on the fixation light. In static testing the examiner moves the pointer to the desired testing location, manually opens and closes a shutter delivering a light stimulus to the inside of the dome. Using the pointer as a guide, he then records on the chart paper whether the patient detected (eg. pressed his button) or did not detect the stimulus. A variation on this technique relates to kinetic testing in which the examiner positions the stimulus, holds the shutter open, and moves the stimulus inward (toward the patient's fixation) until a response occurs. The location of the pointer at the time of the patient's response is recorded manually on the chart paper as the patient's threshold point. The marked positions of all of the tested points on the chart define the field of vision of the eye tested for the size and luminance of the stimulus used. The size and luminance of the stimuli are adjustable by means of controls on the perimeter.
When carefully conducted the tests are accurate and definitive. Practically, however, a high degree of skill is required of the examiner and extensive training is necessary to insure sufficient test standardization to insure that the data obtained is valid and comparable to previously obtained data. Traditional visual fields testing devices leave a great degree of the responsibility for the conduct of the test in the hands of the technician. Moreover, omissions of stimulus positions, use of incorrect stimulus levels, varying duration of shutter operation, inappropriate rate of movement of the stimulus in kinetic testing and other procedural faults, whether occurring inadvertently or not, significantly reduce the clinical value of the test. Because the procedure is time consuming, tedious and boring, employee turnover in this area is often great, further compounding the problem of technician training. In some areas of the world the physician performs the visual fields testing himself, finding this the only way of insuring proper execution of the test.